The wide utilization of cellular and wireless devices drives the rapid development of radio frequency (RF) technologies. The substrates on which RF devices are fabricated play an important role in achieving high level performance in the RF technologies. RF complementary metal-oxide semiconductor (CMOS) and RF micro-electro-mechanical system (MEMS) technologies fabricated on silicon on insulator (SOI) substrates have attracted extensive attention in recent years. Fabrications of these RF technologies on SOI substrates may benefit from low cost of silicon materials, a large scale capacity of wafer production, well-established semiconductor design tools, and well-established semiconductor manufacturing techniques.
Despite the benefits of using SOI substrates and conventional semiconductor manufacturing techniques for RFCMOS and RFMEMS fabrications, it is well known in the industry that a silicon handle layer in the SOI substrate has two undesirable properties that limit the SOI's usage in critical, RF large signal applications: harmonic generations and low resistivity values. By using SOI substrates in RF fabrications, an interface between the silicon handle layer and an adjacent insulator layer will generate significant harmonic signals. RF signals of power levels employed in cellular and other wireless applications are RF signals coupled to this interface and therefore experience significant deterioration with the creation of unwanted harmonic and intermodulation products. Such spectrum degradation causes a number of significant system issues such as unwanted generation of signals in other RF bands which the system is attempting to avoid. In addition, unlike gallium arsenide (GaAs), the silicon handle layer does not have high resistivity values. The relatively low resistivity encountered in the silicon handle layer also limits the performance and quality factor of passive components, such as inductors, transmission lines, and couplers, by the generation of unwanted RF current loss in the silicon handle layer.
Accordingly, there remains a need for improved substrate designs to benefit from high fabrication yields and low cost of silicon manufacturing without bearing deleterious harmonic generations. In addition, there is also a need to enhance the resistivity values of the substrates in final RF products.